1. Field of the Invention
The present invention relates to a paper diaper including an indicator, and more specifically to a paper diaper providing both safety and practicality.
2. Description of the Prior Art
There is an indicator of a paper diaper, the color of which is changed from one color to another color (see, for example, Patent Document 1).
Such an indicator has the following structure. On an inner side of a back sheet of the paper diaper, a color layer formed of water-insoluble blue ink is provided; and on the color layer, a color change layer formed of water-soluble yellow ink is provided. Before being wetted with urine, the indicator appears green as a result of the blue color of the color layer and the yellow color of the color change layer being combined. However, when the indicator is wetted with urine, the yellow ink of the color change layer is dissolved, and the color of the color change layer is faded. As a result, the indicator appears blue as a whole. The indicator indicates that the wearer of the paper diaper has urinated by this color change from green to blue. The yellow ink which has dissolved in urine is absorbed to an absorption body covered with the back sheet.
However, all the yellow ink is not quickly absorbed to the absorption body, and thus a peripheral portion of the indicator appears to be blotted with the yellow ink and the external appearance of the paper diaper is unpleasant. For this reason, the indicator is conventionally formed to be shaped as one thin line.
With such an indicator formed of one thin line, especially when the amount of urine is small or when the position of urination is shifted from the position of the indicator, it may not be clearly determined that the color has changed. Since the indicator is narrow, the amount of urine is not easily estimated from the color change of the indicator. In such a case, the paper diaper is unnecessarily replaced.
Patent Document 2 proposes an indicator which uses color-developing ink, instead of the indicator, the color of which is changed. The color-developing ink is formed of a combination of an ink binder, a colorant, a pH adjusting agent, solvent-containing ink, and a developer. The colorant, which is colorless owing to the function of the pH adjusting agent, is colored by the function of the developer which is formed of a basic substance. The indicator formed of such ink has a hue thereof changed by cleavage of a lactone ring of the developer and thus does not provide an unpleasant external appearance due to the indicator being blotted.
Examples of usable basic substances include ammonia, sodium bicarbonate, calcium carbonate, magnesium carbonate, magnesium aluminosilicate, magnesium oxide, calcium oxide, calcium hydroxide, magnesium hydroxide, calcium silicate, magnesium silicate and the like. Much care is needed in selecting a basic substance so that there is no harm to infants, who are delicate. A basic substance merely has a function of changing the color, and one basic substance cannot develop a plurality of colors. In addition, the indicator is not colored instantaneously, but requires several minutes to be colored.